In modern terminals for mobile communication, use is increasingly being made of adaptive matching circuits for tuning the antenna. Such matching circuits are intended to be used to ensure that the terminal always has optimum antenna matching even in changing environments which influence the impedance of the antenna, the antenna matching being used to improve the transmission and reception operation of the terminal by saving transmission power and improving the reception quality. For this purpose, a detector for determining the reflected power is connected to the respective signal path, which is a transmission path or a hybrid transmission/reception path.
The reflected power is a measure of the mismatch and can and is intended to be minimized by the matching circuit. For this purpose, a control signal is produced from the value of the reflected power, as determined by the detector, using a controller and is used to adjust the adaptive matching network. In known front end modules which are used in such terminals for mobile communication, such a detector is typically used on the principal antenna in order to match both the antenna and the transmission/reception device to a changing environment in combination with an adaptive matching network.
The problem with known matching circuits is that the reflected power can be ascertained only in transmission mode. However, a reflected but immeasurable power also leads to a loss of signal intensity and signal quality in reception mode. This means that the antenna tuner or the relevant matching network can, strictly speaking, be matched only to the transmission signal, while in the reception band (Rx band), which is different from the transmission band (Tx band), it is possible to match the reception mode only “blind” and particularly using predetermined stored empirical values.
Furthermore, when measuring the reflected power, it is not possible to obtain any information about the direction in which matching needs to be performed. Therefore, it is difficult to set optimum matching, particularly when the control signal is used to perform “matching” in the wrong direction, so that ultimately the mismatch is not eliminated or is not eliminated immediately. It is also disadvantageous if the detector and the controller connected thereto detect only a secondary minimum for reflected power, which secondary minimum it is unable to overcome using its closed-loop control algorithm, and it thus misses the optimum matching value.